Incident documentation/2021-10-29 graphite

document status: in-review

Summary

Impact: The backfill process for Graphite metrics silently failed during the Bullseye migration. A subset of metrics experienced loss for data points before October 11th 2021

The process of reimaging a Graphite host is as follows:

1. reimage host
2. let metrics flow for a few days to validate the host is working
3. backfill the rest of the data (online, no downtime) from the other Graphite host following https://wikitech.wikimedia.org/wiki/Graphite#Merge_and_sync_metrics

During the Bullseye migration the backfill process failed (undetected) for a subset of metrics, leading to metric data loss once the Bullseye migration was complete (i.e. graphite2003 first and then graphite1004 were reimaged and put back in service)

Timeline

All times in UTC.

• Oct 11 11:45 reimage of graphite2003 https://phabricator.wikimedia.org/T247963#7416306
• Oct 18 11:30 backfill of graphite 2003 https://phabricator.wikimedia.org/T247963#7435382
• Oct 19 failover from graphite1004 to graphite2003
• Oct 21 10:24 reimage of graphite1004 and data backfill from graphite2003 https://phabricator.wikimedia.org/T247963#7447084
• Oct 25 failover from graphite2003 to graphite1004 https://phabricator.wikimedia.org/T247963#7455052
• Oct 26 16:50 report of missing metric data in https://phabricator.wikimedia.org/T294355

Detection

Some Grafana dashboards backed by Graphite showed partial data (starting Oct 11 or Oct 21) for a subset of metrics, as reported by Lucas Werkmeister in https://phabricator.wikimedia.org/T294355

Conclusions

The whisper-sync backfill process is not as reliable as previously thought, no visible errors were logged and/or detected.

What went well?

• Only a subset of metric files experienced data loss

What went poorly?

• The data loss was not detected by automated means or during spot-check validation

Where did we get lucky?

• Only a subset of metric files experienced data loss

How many people were involved in the remediation?

• 1 SRE (Filippo Giunchedi)

Links to relevant documentation

• https://wikitech.wikimedia.org/wiki/Graphite#Merge_and_sync_metrics

Actionables

• Understand the feasibility (and need) to back up a small subset of important metrics https://phabricator.wikimedia.org/T294355#7464552
• Revise the backfill procedure to be more robust in the face of similar failures in the future (e.g. run a full rsync first, then backfill only the gap) https://phabricator.wikimedia.org/T296295
• Perform validation post-sync / post-backfill to check the number of datapoints across all metric files is roughly in sync between hosts https://phabricator.wikimedia.org/T296295
• Continue (and speed up) the Graphite retirement plan https://phabricator.wikimedia.org/T228380